The manufacture of both curved and straight scissors has been a persistent problem in robotic surgical tools. Two design considerations for the manufacture of robotic surgical scissors are performance and corrosion.
The ability to cut delicate and tough tissue in a human body is limited because the force available to the robotic surgical scissors jaw blades is limited. For most robotically assisted surgery, small scissors are used because the surgical instruments must pass though small ports into a patents body. The robotic surgical scissors are generally connected to a narrow shaft allowing a surgeon to manipulate the scissor inside the patient's body. The size of the scissors, and the length and narrowness of the shaft constrain the amount of power that can be applied to the scissor blades. Limitations on power can be alleviated somewhat by a sharper cutting edge. An exceptionally sharp blade therefore is preferable in robotic surgical scissors. Additionally, robotic surgical scissors are preferably designed to cut in a controlled manner over a range of motion of the jaw blades.
Sharper blades generally lead to better scissors. Sharper blades may be achieved by using a harder metal that can be sharpened to and hold a finer edge. The type of metal alloy selected to manufacture a blade and the sharpening technique (e.g., honing and stropping) employed to sharpen an edge can improve scissor performance.
Sharp robotic surgical scissors typically cut better than dull robotic surgical scissors. Hard metal alloy materials, such as Martensitic stainless steels, may be used to form the jaw blades as it can be ground and honed to a fine edge. In a more general sense, martensite is used to describe crystals which have changed geometries following rapid cooling. The term is applied to describe metals or metal alloys that have been tempered in a heating process and then rapidly cooled, usually by quenching. Martensitic stainless steel is exceptionally hard and can hold a keen edge but at the same time they are extremely brittle. A pair of surgical scissors formed out of Martensitic stainless steel may shatter if dropped on a hard surface.
Honing an edge onto a blade can be an intricate and laborious process. In a honing process, a first side of a blade edge is applied to a grinding wheel or other abrasive device until a burr forms. The presence of the burr means that the steel is thin enough at the top so that it is folding over slightly. Once the burr is formed on a first side, the process is repeated on the second side of the blade edge opposite the first side. Honing a blade in this manner typically results in a very thin and razor sharp edge. The very thin and razor sharp edge is often referred to as a wire edge. The wire edge however is fragile and will likely break off during the very first use, leaving an extremely dull blade. The reason for this structural weakness is that the wire edge is too thin and there is not enough metal left at the edge for strength. After honing, a stropping process may be used to strengthen a wire edge.
Stropping is essentially the removal of the wire edge by brushing the edge of a newly honed blade over a softer surface usually impregnated with some abrasive compound such as stropping paste or green chromium oxide. Jewelers rouge is also acceptable. Leather is a typical strop, often used by barbers, being just firm enough to strengthen a wire edge without dulling it completely.
The honing and stropping process may be complex and labor intensive if performed manually. In sharpening blades on a commercial scale, sophisticated machinery is often used to handle the blades as they are honed. Manufacturing equipment holds the blades in contact with a grinding surface at a predetermined angle with a predetermined pressure. The predetermined angle and pressure may be maintained consistently along the entire blade edge to achieve a uniform sharpness. Generally scissor blades that are flat and uniform in size are easier to sharpen than oddly shaped scissor blades with non uniform proportions.
By their very nature, surgical scissors are prone to stress-corrosion, specifically chemically assisted degradation. A scissor cuts at the intersection of the two blades. In more effective scissors, the blades tightly press against each other. It is difficult for a very sharp scissor blade to cut properly if the fastener (e.g., rivet, bolt, or screw) that couples the blades together is loose. In some scissor designs, the scissor blades slightly curve inward to facilitate contact. The constant contact between scissor blades causes stress fields in the blades themselves, specifically at the edges. A stress field in the blades of the robotic surgical scissors is due to the scissors blades pressing against each other.
Harder metals can hold a sharper edge but tend to be more brittle. Moreover, the harder the cutting blades are made to provide sharp edges in the robotic surgical scissors, the more vulnerable they are to corrosion caused by the presence of a stress field. The hard brittle metal used in most surgical blades is particularly vulnerable to stress related chipping and cracking.
Corrosive materials may be present during surgery. For example, saline is often used during surgery to irrigate tissue in the surgical site. Saline may promote corrosion of the materials used in the manufacture of robotic surgical scissors. Typical rust inhibitors, such as oil based lubricants, are not suitable for use in surgery because of the sterile nature of the surgical site and the risk of contamination and infection. Even though surgical tools are often made of stainless steel because of its corrosion resistant properties, the surgical tools may be experience a corrosive effect on its materials.
Corrosion at the sharpened edge of a blade can result in a duller blade. Additionally, corrosion and the stress field may further weaken the already brittle wire edge causing small flakes of metal to break off inside a patient's body. The corrosive effect on the materials and the stress field may eventually cause a jaw blade to become unusable or fail, such as by cracking (stress fractures) and breaking and falling off from the robotic surgical scissors. A material failure in end effectors of a robotic surgical tool, such as a scissors blade, is undesirable when surgery is being performed within a patient.
One solution to reduce the effects of corrosion and stress fractures in surgical instruments is to replace them often. Newer scissors will be sharper and have less time to corrode. The problem with this simple solution is the cost of replacing robotic surgical tools. Ideally surgical scissors would be replaced after every surgery but doing so is expensive.
Each jaw blade of robotic surgical scissors are typically manufactured using a single material to form both a cutting blade and a drive hub together as one piece. Typically, a cable directly attaches to the drive hub of each jaw blade to lever and pivot the jaw blades around a fulcrum. One example of such a scissors mechanism is used in robotic surgical tools with the DAVINCI™ robotic surgery system made by Intuitive Surgical, Inc. of Sunnyvale, Calif.
As both the drive hub and the cutting blade are typically manufactured together using the same material, they may be manufactured together as one in the same process, such as through a forging process. However by being manufactured together using the same material during the same manufacturing process, a materials compromise may be made between the design requirements for the drive hub and the design requirements for the cutting blade.
The metal alloy used for a typical surgical scissor must be hard and capable of holding a fine edge. Such a metal alloy is also typically brittle. The drive element on the other hand, does not require the same material property.
Moreover manufacturing the drive hub and the cutting blade together as one piece using the same material during the same process forms a complex shape that may increase manufacturing costs. The complex shape also makes it more difficult to sharpen the cutting edge.